1. Field of the Invention The present invention relates to a method of processing seismic reflection data for attaining a better knowledge of the structure of the geological layers of an environment to be explored.
2. Description of the Background Art
In very general terms, it is considered in geophysics that the subsoil is constituted from a stack of geological layers having different characteristics which are organized in space according to a certain geometry.
In order to define the geometry of the layers of the subsoil, subsoil exploration specialists, in particular geophysicists engaged in oil exploration, use a special technique called "seismic reflection" which consists of transmitting acoustic signals from different points of the surface of the ground, called transmitting points, and thereafter receiving at different points on the surface of the ground, called receiving points, the acoustic signals after they have been propagated in the subsoil and have been reflected at the individual acoustic boundaries which constitute the limits between different geological layers and are therefore called reflectors. Each of the elementary recordings, which are a function of time and are associated with a given transmitting point and a given receiving point, constitutes what is called a "trace".
For a given transmission, a simultaneous recording is made at different receiving points, over a predefined time, of the signals reflected by the different reflectors of the subsoil. This technique allows traces carrying redundant information to be grouped in collections.
The recorded traces are processed in order to obtain individual images of the subsoil called "seismic sections", which can be assimilated as vertical cross-sectional planes of the subsoil on which the reflectors appear as lineations superimposed on each other. The processing technique commonly used consists of summing between the traces of each collection after having applied a certain number of corrections to them and thus obtaining a "sum trace" associated with a particular point of the surface. The choice of the collections and the corrections are based on simplifying hypotheses relating in particular to the geometric structure of the layers of the soil, the latter being assumed to be horizontal and homogeneous. In this case, the reflection point of the signal is located on a vertical passing through the mid point of the segment joining the transmitting and receiving points on the surface of the ground. Each of the collections will then be constituted by all of the traces associated with a same mid point. To each of the traces there is applied at least one correction called the "dynamic correction" whose purpose is, starting from expansion of the time scale, to correct the effects of obliqueness of the transmitter-reflector-receiver paths, which are different for each of the traces of a same collection, in order to obtain the trace which would be obtained directly on the ground if the transmitter and the receiver were merged at the same point on the surface of the ground.
All of the processings currently applied in a routine manner in seismic reflection consist in concentrating the redundant data carried by collections in a single sum trace, by horizontally summing all of the traces of a same collection. A sample of the sum trace is the result of the summing, weighted or not weighted, of the N samples at the same time each belonging to a different elementary trace, N being the number of these traces.
The assumption that the layers are horizontal, if it is not fully justified, leads on the one hand to a considerable geometric distortion of the image of the subsoil, because, by operating in multiple cover, data originating from points which are not on the vertical from the mid point are positioned on this vertical, and on the other hand to a lowering of resolution, because data is summed which comes not from the same point, which is the case if the assumptions are true, but from different points of the subsoil, which has the effect of degrading the signal. This distortion and this degradation can be considerable to the point of falsifying the interpretation which is then made of these seismic sections, particularly in the case of a disturbed subsoil geometry or of fine structures for which a high resolution is necessary. Various processings exist, called migrations, all of which have the object of restituting an image of the subsoil which is as exact as possible, by replacing the various received data in the correct position. Extensive literature exists on the various migration methods and can be referred to for more information. A differentiation will however be made between the migrations called "after sum" which are performed on the sum traces and which take into account the geometric distortion alone, and the "before sum" migrations. The before sum migrations are performed on each of the traces of each of the collections of elementary traces before summing, and theoretically have the effect of putting into phase, i.e. of allocating a same depth or same time to signals which relate to the same reflection point.
A collection of elementary traces on which a before sum migration has been applied can be considered as an elementary "mini-section" of the subsoil in a plane whose coordinate axes are distance X and time or depth T over which the signals relating to a same reflector are horizontally aligned. The quality of such a migration applied to real data in fact heavily depends on knowledge of the speed of propagation of the wave in each of the layers of subsoil, called the "interval speed". Errors in the interval speed cause distortions of the alignments, which because of this, are no longer horizontal such that the signal obtained after horizontal summing is degraded, this degradation being able to lead to errors or difficulties in interpretation.
Another object of the summing in seismic reflection is to improve the signal/noise ratio, the summing reinforcing the amplitude of the significant signals present in each of the traces, theoretically in phase, and attenuating, by destructive summing, the random noise from one trace to another.
This object will obviously not be achieved if the signals are not in phase. A summing carried out in poor conditions, i.e. following a line corresponding to a theoretical alignment and not to a real alignment, has the effect of reducing the amplitude of the sum signal and the apparent frequency. If the line along which the summing is performed is distorted, to take account of these phase shifts of signals from one elementary trace to another, results in that these effects do not occur and the sum signal obtained is an optimum signal.